Coronary artery bypass graft surgery is a typical procedure for treatment of chest pain or heart attack. During the operation veins are taken from another part of the body and used to bypass obstruction in the coronary arteries that surround the heart and supply blood to the heart muscle. Blood flow to the heart muscle is restored by use of the grafted vessels to areas where blood was reduced by obstructions in the coronary arteries. To keep the heart muscle functioning with a constant supply of blood, the bypass grafts must remain open.
If an inadequate supply of blood reaches the heart muscle, ischemia, or tissue death can occur. The heart muscles contract and relax during the blood pumping cycle. Upon contraction the heart muscle thickens and upon relaxation it thins. If the myocardial tissue (heart muscle tissue) is not supplied with sufficient blood and becomes ischemic, muscle contraction may be reduced or stop completely. The other healthy tissue around the ischemic tissue will continue to contract pulling and thinning the ischemic tissue. The thin ischemic tissue may bulge under pressure and an aneurysm can occur.
In monitoring the postoperative bypass patient attention should be directed not only to blood flow in the coronary arteries and the bypass graft, but also the function of the heart muscle supplied by the graft. The left ventricle of the heart pumps blood to the extremities of the body and to the coronary arteries which supply blood and are often the focus of bypass surgery. If there is ischemia in the myocardium of the left ventricle, the detection will indicate graft failure or drug evaluation. Early detection of ischemia is important in planning therapy to avoid irreversible damage and aneurysms.
Small biological sensors haven been developed which can be used to monitor responses during surgery. Some sensors are adapted for implantation in the patient and remain postoperative to constantly monitor function. Blood flow sensors have been developed including electromagnetic type flow meters and ultrasonic Doppler transducers consisting of a single piezoelectric crystal acting as an ultrasonic transmitter and receiver.
The implantable Doppler transducers used as blood flow velocity sensors are very small and can be used in a probe to monitor continuously the blood flow of vessels in a patient for a period of time, postoperative or otherwise. The sensor or probe must be secured to the vessel to assure proper flow velocity measurements. Examples of Doppler transducers used in flow probes and the technique has been reported by Payen, D. et al. "Comparison of Preoperative and Postoperative Phasic Blood Flow in Aortocoronory Venous Bypass Grafts by Means of Pulsed Doppler Echocardiograph with Implantable Microprobes; Circ; Vol. 74 (Suppl. III), pp. 61-67 (1986); Svenning, J. L. et al, "Continuous Monitoring of Cardiac Output Postoperatively Using an Implantable Doppler Probe, Scand. J. Thor. Cardiovasc. Surg., Vol. 20, pp. 145-149 (1986); and Baudino, U.S. Pat. No. 4,541,433 issued Sept. 17, 1985.
Several methods have been described to provide measurement of regional myocardial function. Sensors are used to measure the thickening of the myocardial tissue. For continuous assessment of regional dimensions of the left ventricular wall, the distance was measured between a pair of ultrasonic sensors plunged into the myocardium to a depth of 7 mm from the epicardium, 10 mm apart. The myocardium is pierced twice. Hill, R. C. et al "Perioperative Assessment of Segmental Left Ventricular Function in Man," Arch. Surg., Vol. 115, 609 (1980).
Myocardial thickening-sensors have been developed by our laboratories which are attached to the epicardial (outer) surface of the heart muscle. Only one sensor is needed and invasion into the myocardium is not necessary for use. The sensors are small piezoelectric crystals which detect thickening of the myocardium as more fully discussed in the cited references. Hartley, C. J. et al, "Intraoperative Assessment of Regional Myocardial Function in Man," Proc. 8th Int'l. Conf. of the Cardiovasc. System Dynamics Soc'y., Osaka, Japan, Vol. 9 (1-3) pp. 13 (1987); Zhu, W. X. et al., "Validation of a single crystal for the measurement of transmural and epicardial thickening", Am. J. Physiol., Vol. 251, pp. H1045-H1055 (1986); Hartley et al., "Doppler measurement of myocardial thickening with a single transducer", Am. J. Physiol., Vol. 245 (Heart Circ. Physiol. 14), pp. H1066-H1072 (1983).
The present invention is a multifunctional probe with one sensor to measure blood flow in the coronary artery and another sensor to measure myocardial thickening. The probe containing both sensors is implantable and useful for intra and postoperative monitoring. The small probe can be removed from the closed patient several days after surgery through a small opening in the patient's chest. The removal of the probe requires no invasive surgery. During the monitoring process the lead wires extend through this small opening. When the probe is no longer needed gently traction is placed on the tube and lead wires and the probe is removed.
The sensors are contained in a probe body of biocompatible material such as silicon rubber which is nonreactive to body tissues and fluids. The placement of the sensors in the probe body can be in any configuration and spacing desired. For use in postoperative monitoring of both coronary artery blood flow and ventricular thickening, the spacing would typically include a crystal placed over an epicardial coronary artery to measure blood flow velocity spaced about 4 mm to about 8 mm from a crystal which measures regional myocardial function.
The probe body with a blood flow velocity sensor and a myocardial thickening sensor is lightly sutured less than 2 mm to the outer surface of the epicardium. The blood flow velocity sensor is positioned over the coronary artery without any need for dissection of the artery or the adjacent tissue. The thickening sensor generally is placed over the myocardium which is supplied with the blood from the coronary artery and the bypass graft to measure the thickening of the heart muscle. The probe body terminates in a flexible tube through which the lead wires extend. The tube extends outside the patient's body similar to a surgical drainage tube and the lead wires are connected to the electronic monitoring equipment.